Field of the Invention
The invention relates to a device for cooling the surface of a blanket of a printing unit cylinder and, more particularly, in a dry offset printing unit of a rotary printing press which has at least one plate cylinder and at least one blanket cylinder, the printing unit being equipped with a cooling circuit wherein a fluid circulates.
Published European Patent Document EP-0,480,230 describes a thermal regulator or control for a printing plate disposed on a plate cylinder for dry offset printing. The thermal regulator is formed with a chamber for sweeping the plate with cold air, wherein fans and a refrigerating device supplied with refrigerant from a refrigerating source or plant on the outside are provided. The cooling of the refrigerant is regulated or controlled as a function of a temperature determined by the regulator or control. The construction of this heretofore known device is bulky because a coil for the refrigerant requires a path which is of such great length as to be able to cool the air.
Published European Patent Document EP-0,553,447 describes a system for cooling a printing plate of a printing press. A transverse member for blowing in cooling air is arranged lengthwise across the surface of the plate in order to keep the temperature thereof at a desired value. The transverse member houses at least one heat exchanger and at least one blower as well as at least one air reflux duct, all of which together form a cooling-air circuit. The air blown in at the surface of the plate is recycled by this circuit towards the intake side of the heat exchanger and is discharged again by the blower over the surface of the plate, possibly being mixed with fresh air after having passed through the heat exchanger. The transverse member for blowing in air represents a compact system, which does not consume very much energy, for cooling the surface of the plate. However, a drawback exists in that droplets of condensed moisture undesirably reach locations on the printing plate.
Published European Patent Document EP-0,627,308 describes a device for cooling rotary members of printing units. It has an in-blowing device including an internal circuit for recirculating the cooling air which has been deflected by the rotary member to be cooled. A cold-air generator is situated outside the in-blowing device and is connected thereto by an air inlet pipe. Energy is economized in the production of cold air, and the cold-air inlet pipes can have a relatively small cross section because a small amount of this air is all that is required. In this case too, a drawback of such a device is that droplets of condensed water may reach regions on the printing plate where they are highly undesirable and run the risk of considerably reducing the quality of the prints produced.
Published European Patent Document EP-0,638,418 describes a system for cooling cylinders of a printing press. It has at least one compressed-air inlet pipe including at least one orifice for blowing in cold air against a cylinder to be cooled. At least one recirculation circuit which is separate from the cold air arriving through the compressed-air pipe is provided, on the one hand, for sucking up the air directed through the in-blowing orifice onto the cylinder by a blower which is included in this circuit and, on the other hand, for blowing it back in onto the cylinder in parallel with the cold air. In this way, the temperature of the cold air can act upon the cylinder without a preliminary change in temperature. The cold air deflected by the cylinder is returned to the latter in order to participate in the cooling thereof.
Published European Patent Document EP-0,652,104 describes a dry offset printing unit. The dynamics of the printing unit are to be improved, and an accumulation of ink on the blanket is to be avoided. To this end, the plate cylinder supports a printing plate in the form of a sleeve, and/or the blanket cylinder supports a transfer plate in the form of a sleeve.
The printing and/or transfer plate may be slipped over the corresponding cylinder or cylinders via an opening formed in a cheek or jaw of the printing press which can be uncovered.
It should be noted that the maintenance of a given temperature is a crucial factor in dry offset printing.
Rapid fluctuations in temperature may have catastrophic consequences in dry offset printing. The heating-up which results from the operation of the printing units may be compensated for by internally cooling the compartments of the frame or the inside of the cylinders and no longer raises any problems. It has been discovered that a crucial parameter is the surface temperature of the blanket or of a sleeve forming a blanket. The work of compressing the layers of rubber causes the surface of the blanket to heat up very quickly, whereas it cools down quickly when the printing unit stops. Attempts at combating the heating-up by internally cooling the blanket cylinder with a cold-water circuit have been made. However, because the structure of the blanket has an insulating effect, the temperature of the fluid coolant has to be reduced considerably. Condensation may therefore occur on the end surfaces of the cylinder even though the blanket surface remains barely cooled.